1. Field of the Invention
The present invention relates to an improved side tone preventive circuit for telephones, namely, a circuit for preventing the output of the transmitter on the speaker's side from being transmitted (output) to the receiver on the same side.
2. Description of the Related Art
Most of the present telephones are connected to a two-wire telephone circuit, which is divided into a circuit on the receiver's side and a circuit on the transmitter's side by a two-wire/four-wire converter consisting of an impedance bridge, and a signal that is transmitted. When the balance of the impedance bridge is lost, the output of the transmitter on the speaker's side is transmitted to the receiver on the same side, so that the voice of the speaker is output from the receiver, thereby disturbing good communication. To prevent such a side tone in a conventional telephone, the impedance balance of the two-wire/four-wire converter is controlled.
The side tone preventive circuit in a conventional telephone will be explained with reference to FIG. 10. In FIG. 10, a telephone office 100 has an exchanger for connecting a plurality of telephone circuits. The exchanger has one trunk circuit 10 per telephone circuit connected thereto. The telephone circuit from the telephone office 100 is connected to a telephone 300 installed in a home or the like through telephone lines 200.
Each telephone line 200 is composed of a resistance of the telephone line itself, which changes in accordance with the diameter of the line, the distance between telephone 300 and trunk circuit 10, and the like; and a stray capacitance which is parasitic on the resistance, as schematically shown in FIG. 10. Telephone lines 200 can therefore equivalently represented by resistors R.sub.L, capacitors C.sub.L and the like, as shown in FIG. 10.
The circuit impedance Z.sub.L seen from the side of telephone 300 is determined by the impedance of trunk circuit 10 and the telephone lines 200 seen from side of the connecting terminals L.sub.1 and L.sub.2 of telephone 300.
Telephone 300 has an internal circuit 12 connected across connecting terminals L.sub.1 and L.sub.2 so that internal circuit 12, telephone line 200 and trunk circuit 10 are connected with each other in parallel. Connecting terminal L.sub.2 is grounded, and balance impedance elements R.sub.9 and R.sub.10 for forming a bridge circuit are connected in series to connecting terminal L.sub.2. One end of a balance adjustment circuit 14 is connected to connecting terminal L.sub.1, and the other end thereof is connected to one end of balancing resistance R.sub.10. In this way, the balancing resistances R.sub.9 and R.sub.10, balance adjustment circuit 14 and the circuit consisting of internal circuit 12 and the external circuit comprising telephone line 200 and trunk circuit 10 which are connected with each other in parallel constitute an impedance bridge.
A receiver 16 is provided between connecting terminal L.sub.2 and the junction of resistor R.sub.10 and balance adjustment circuit 14, and a transmitter 18 is provided between the junctions of the resistors R.sub.9 and R.sub.10 and the connecting terminal L.sub.1.
Therefore, if the impedance thereof satisfies the following relationship: EQU (Z.sub.L // Z.sub.SN).R.sub.10 =Z.sub.BN.R.sub.9
the impedance bridge satisfies the balancing condition, so that the output of the transmitter 18 is not transmitted to the receiver 16. Z.sub.L // Z.sub.SN represents impedance of Z.sub.L and Z.sub.SN connected in parallel.
In such a conventional impedance bridge, if the above-described balancing condition is satisfied, a sufficiently good side tone prevention is achieved. However, the circuit impedance Z.sub.L seen from the side of telephone 300 greatly changes in accordance with the diameter of telephone line 200 and the distance between telephone 300 and trunk circuit 10. A line having a diameter of 0.32, 0.4, 0.5, 0.65, 0.9 mm or the like is typically used as telephone line 200, and the loss of signal level changes by about 0 to 7 dB in accordance with the length, of telephone line 200. Therefore, the circuit impedance Z.sub.L greatly changes depending upon the state in which telephone 300 is installed. However, since balance adjustment circuit 14 in a conventional telephone 300 is composed of a stationary CR circuit net, it cannot respond to the impedance Z.sub.L of the telephone circuit which greatly changes.
It is at only one point in the greatly variable impedance Z.sub.L of the telephone circuit that the impedance bridge satisfies the balancing condition, and the impedance bridge is not balanced under other conditions. Therefore, when the circuit impedance Z.sub.L changes, it is impossible to achieve good side tone prevention.